Regeneration Behaviour of Platinum Group Metals Promoted Cracking Catalysts

D.L. Trimm et al. (Editors), Catalysts in Petroleum Refining 1989 1990 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

565

REGENERATION BEHAVIOUR OF PLATINUM GROUP METALS PROMOTED CRACKING CATALYSTS P. STEINGASZNER, A.

szucs & L. MERI

Department of Chemical Technology, Technical University of Budapest, H-1521 Budapest, Budafoki 6 t 8. (Hungary)

ABSTRACT Regeneration behaviour of platinum group metals promoted cracking c a t a l y s t s has been i n v es t i g at ed by means of carbon monoxide oxidation a c t i v i t y tests and high s e n s i t i v i t y thermoanalytical methods. E f f e c t s of physical and chemical f a c t o r s - such as thermal and hydrothermal tre a tm e nt, t r a n s i t i o n metals c o n t e n t , presence of sulphur compounds as hydrogen sulphide and sulphur dioxide - on t h e carbon monoxide oxidation a c t i v i t y of s e v e r a l c o m r c i a l cracking c a t a l y s t s have been determined. Computer aided d i f f e r e n t i a l thermogravimetry has been used t o d es cr i be t h e dynamics of coke oxidation on promoted cracking c a t a l y s t s and t o determine k i n e t i c c o r r e l a t i o n s i n func tion of promoter concentration.

INTRODUCTION Catalytic

cracking

petroleum r e f i n i n g oils

into

feed

is

particles, catalyst

is

one

industry

of

for

the

most

important

processes

i n the

t h e conversion of heavy f r a c t i o n s of crude

valuable l i g h t e r products. During t h e cracking cycle a part of t h e converted

to

rendering can

he

coke which

deposits

on the surfa c e of t h e c a t a l y s t

them less a c t i v e f o r cracking. Cracking a c t i v i t y of t h e

r es t o r ed

by removal of t h e coke by burning with c ontrolle d

of air. The cracking a c t i v i t y of t h e regenerated c a t a l y s t strongly on t h e coke remaining a f t e r r eg ene ra tion, t h e r e f o r e , e s p e c i a l l y with r e g u l a r channel s t r u c t u r e z e o l i t i c cracking c a t a l y s t a coke l e v e l of less than 0.1% is m d a t o r y i n order t o g e t best r e s u l t s . Proper coke removal can be e f f e c t e d by c ar r y i n g out t h e regeneration at high temperature ( r e f . l > , i n two s t e p s ( r e f . 2) and/or by using coke oxidation improvers e i t h e r incorporated i n t o t h e cracking c a t a l y s t , or using them i n a concentrated form, added t o the c a t a l y s t ( r e f s . 1 , 7-61. Members of t h e platinum group of t h e Pe riodic Table of Elements a r e e x c e l l e n t oxidation c a t a l y s t s , and, according t o t h e pa te nt l i t e r a t u r e , t h e i r presence - alone or i n combination - on t h e cracking c a t a l y s t s i n ppm amounts results i n near complete combustion of t h e coke, preventing damaging addition

depends

566

afterburns

in

regenerator

vessel,

giving

lower coke on t h e regenerated c a t a l y s t and

coke

cracking from

make,

however

monoxide

ability

b e t t e r h e a t economy and - due t o

higher g a s o l i n e y i e l d s with lower gas

composition d a t a on c o m e r c i a l l y a v a i l a b l e promoted

are not a v a i l a b l e . The advantages mentioned r e s u l t m i n l y

catalysts

the

-

of

group metals t o c a t a l y z e oxidation of carbon

platinum

formed during r e g e n e r a t i o n t o carbon dioxide i n t h e dense

primarily

c a t a l y s t phase. I n d u s t r i a l experience has shown t h e coke combustion a c t i v i t y of oxidation improvers

to

oxidation

promoters

hydrotermal

decline

during used

repeated

with

them

use. are

As

cracking c a t a l y s t s and t h e

s u b j e c t e d t o repeated thermal and

c y c l e s , oxidative and reducing atmospheres i n t h e r e g e n e r a t o r and

r e a c t o r r e s p e c t i v e l y , and a d d i t i o n a l l y metals from t h e feed - n i c k e l , vanadium and i r o n - accumulate on them, f u r t h e r , d i f f e r e n t sulphur compounds, mainly as and hydrogen sulphide i n t h e cracking r e a c t o r and as sulphur oxides i n

thiols the

regenerator

investigations

influence

combustion a c t i v i t y ( r e f s . 1, 7, 8 ) ,

coke

were s t a r t e d t o show t h e q u a n t i t a t i v e e f f e c t s of t h e s e f a c t o r s

on carbon monoxide sensitivity

their

oxidation

activity.

Additionally,

computer aided high

methods have been used t o shed l i g h t on k i n e t i c

thermoanalytical

a s p e c t s of t h e regeneration process. EXPERIMENTAL The

regeneration

effectiveness

carbon monoxide/carbon experiments carbon been in

dioxide

1.) o r

(ref.

monoxide

specified

behaviour

of

promoted

cracking

catalyts

and

the

of oxidation improvers may be s t u d i e d e i t h e r by determining t h e by

ratio

i n f l u e gases from batch r e g e n e r a t i o n

measuring t h e carbon monoxide conversion i n a

and

oxygen

c o n t a i n i n g gas

led

over

the

c a t a l y s t under

conditions

(refs.

5,9). I n t h e p r e s e n t work t h e latter method has

used by feeding a gas stream containing 2 % by volume of carbon monoxide

air i n t o an e l e c t r i c a l l y heated isothermal q u a r t z r e a c t o r containing t h e

c a t a l y s t , at a flow rate of 30 l i t e r s / h o u r . The carbon monoxide content of t h e feed

gas,

infrared

as t h a t

as well analyzers.

The

of

volume

t h e e f f l u e n t has been measured by means of of

the

catalysts

was

6.2

ml

in

all

experiments. Catalyst

AF,

BF,

and

CF, all c o m e r c i a l l y a v a i l a b l e promoted f r e s h

c a t a l y s t s , have been used. Their most important d a t a are compiled i n Table Prior treatment containing

to

at

1.

measurements t h e c a t a l y s t samples have been s t a b i l i z e d by h e a t 500°C

2% by

for volume

2 hours i n air, followed by a pretreatment with air of

carbon

monoxide

at

5OO0C

for

30 minutes.

conversions have been determined at d i f f e r e n t temperatures. Values of carbon monoxide conversion were p l o t t e d a g a i n s t

Subsequently,

carbon monoxide

567

TABLE 1. Data of the catalysts used Catalysts Chemical composition, % by w t .

A1203

28.00 0.20 0.08

Na20 Fe Physical properties Apparent bulk density (g/rnl) Pore volume Surface area

AF

(ml/ g 1 (m2/g)

BF

46.80 0.25 0.76

0.53

0.84

0.50 390

0.27 122

CF

32.00 0.70

0.35

0.85 0.19 200

(Reprinted with pmmission from Studies in Surface Science and Catalysis, Vol.

34, p. 452. Elsevier Science Publishers, B.V.)

temperature and these curves have been used to characterize the carbon monoxide conversion activities of the catalyst samples. The measured carbon monoxide conversion values were corrected by substracting the value of carbon monoxide converted thermally, i.e. in the empty reactor. In order to determine the effect of thermal treatment on carbon monoxide conversion activity, catalyst samples were treated at 7OO0C for 3 hours in air prior to measurements. The influence of hydrothermal effects on carbon monoxide conversion activity was modeled by treatment of the catalyst samples in 100% steam at 75OoC for 6 hours before GO-conversion measurements. The effect of transition metals on carbon monoxide conversion activity was tested by impregnating fresh catalyst sample BF with aqueous solutions of nickel nitrate, ferric nitrate and m o n i u m vanadate, respectively to different concentration levels, drying at 120°C for 3 hours and calcining at 7OO0C for 3 hours before conversion measurements, For the determination of coke combustion kinetics, fresh catalyst BF was mixed with different amounts of a comercially available coke combustion improver concentrate. Laboratory catalytic cracking runs have been carried out with these catalysts at 490-495°C ith a standard FCC feed (vacuum gas oil) in the presence of argon carrier gas. The coked catalyst samples were regenerated in a computer coupled Mettler TA-3000 System type thermobalance in air, with linear temperature rate of 15OC/min over a temperature range of 25 to 700%.

568

RESULTS AND DISCUSSION Effect of thermal and hydrothermal treatment Figure 1. shows the carbon monoxide conversion vs. temperature curves for the catalysts investigated. The samples tested were the following: - fresh catalysts AF, BF, and CF, - equilibrium catalysts AE and BE, obtained from industrial fluid catalytic cracking units using fresh catalyst AF and BF, respectively, - thermally treated fresh catalysts AT, BT and CT, - hydrothermally treated fresh catalysts AH, BH, and CH, where subscripts F, E, T and H refer to fresh, equilibrium, thermally and hydrothermally treated catalysts, respectively. Figure 1. shows that different promoted catalysts behave differently on hydrothermal and thermal treatment. With catalyst A the equilibrium form AE is much more active than the fresh catalyst AF whereas the equilibrated catalyst

BE is less active than fresh catalyst BF. The CO-conversion activity of fresh catalyst BF is improved by thermal treatment and even more by hydrothermal treatment. The CO-conversion activity of fresh catalyst AF is increased by thermal treatment, even more by hydrothermal treatment whereas equilibrium catalyst AE shows the highest activity. Catalyst C shows opposite trends as both thermal and hydrothermal treatment decreases its CO-converting activity considerably. Data in Figure 1 prove that upon hydrothermal treatment CO-conversion activities of promoted catalysts from different manufacturers may either increase of decrease. This finding explains why literature data on the effect of hydrothermal treatment are contradictory (See References 1, 10, 11, 12, 13) Effect of transition metals Equilibrium cracking catalysts contain different amounts of transition metals (nickel, iron and vanadium) deposited on the catalyst surface from feeds, containing metals in the form of organic complexes. Usual levels of these metals on equilibrium catalysts vary from a few hundred up to several thousand ppm, depending on the feedstock processed (refs. 7, 14, 15). Transition metals, being dehydrogenation catalysts impair product composition by enhancing coke make and hydrogen yield, decreasing conversion and gasoline yield (ref. 7). Some authors report that transition metals m y improve regenerability of cracking catalysts (ref.161, but there is hardly any information as to the quantitative effect of these metals on the regeneration behaviour of promoted cracking catalysts.

569

0

200

-.

10.,o oa. C

Fig. 1.

4 00

500

600

Effect of thermal and hydrothermal treatment on comercial promoted cracking catalysts (Reprinted with permission from Studies in Surfme Science and Catalysis, Vol. %., p. 454, Elsevier Science Publishers

B.V.)

570

L

I

1000 Fig.

2.

Effect

of

different

of c a t a l y s t BF.

3000 transition

so00

*

PPm Ni

metals on t h e CO-oxidation a c t i v i t y

571 Our carbon

experimental

monoxide

transition

metal

content

question.

In

empirical

evaluation

order

BF prove t h a t its

d a t a obtained with promoted c a t a l y s t

conversion

activity

generally

decreases

a way s p e c i f i c t o

in

the

with

increasing

t r a n s i t i o n metal i n

compare t h e e f f e c t s of t r a n s i t i o n metal poisoning an

to

method

has

been

used: carbon monoxide conversion v s .

curves were i n t e g r a t e d over t h e temperature range of 240 t o 5OO0C

temperature

t h e a r e a under t h e curve measured with unpromoted c a t a l y s t was assigned a

and

promoter

activity

100%. Carbon poisoned

catalyst

0% whereas f o r t h e promoted c a t a l y s t a value of

of

conversion

curves obtained with t h e t r a n s i t i o n metal

samples were i n t e g r a t e d over t h e same temperature i n t e r v a l

a c t i v i t y was read from a c a l i b r a t i o n diagram. Promoter a c t i v i t y

promoter

and

value

monoxide

values were p l o t t e d i n function of t r a n s i t i o n metal content (see Fig. 2.). Figure with

the

2/a

shows

first

t h a t f o r vanadium t h e promoter a c t i v i t y s h a r p l y drops

few hundred

ppm of metal loading, f l a t t e n i n g out at higher

loadings, up t o 5000 ppm. 2/b shows t h a t with i r o n t h e a c t i v i t y d e c l i n e s less s t e e p l y up t o

Figure

concentrations of cca 4000 ppm; at higher i r o n concentrations

metal

a slight

i n c r e a s e i n a c t i v i t y can be noticed.

As

on

shown

conversion

Figure

2/c,

nickel

behaves d i f f e r e n t l y : carbon monoxide

sharply decreases up t o a metal content of 200 ppm, then

activity

i n c r e a s e s s l i g h t l y up t o 500-1000 ppm n i c k e l content t o a f l a t maximum. Carbon

monoxide

conversion

activities

of

transition

metal

poisoned

c a t a l y s t samples have a l s o been measured a f t e r hydrothermal t r e a t m e n t . Results are

shown

Figure 2 with broken l i n e s . The r e s u l t s show t h a t hydrothermal

on

treatment f u r t h e r decreases t h e a c t i v i t y of t r a n s i t i o n metal loaded c a t a l y s t s , catalysts

loaded

with

6000 ppm metals

cca

lose

their

carbon

monoxide

conversion a c t i v i t y almost completely. The

gradual

concentration simply

decrease

in

cover

the

the

of

case

CO-conversion

of

a c t i v i t y with i n c r e a s i n g metal

i r o n and vanadium suggests t h a t t h e s e metals

catalytically

active

sites,

whereas

with

nickel

the

pronounced v a l l e y on t h e a c t i v i t y v s . n i c k e l concentration curves suggests t h e p o s s i b l e formation of a s u r f a c e a l l o y with d i f f e r e n t c a t a l y t i c p r o p e r t i e s . The slight

increase

of CO-converting a c t i v i t y found at high i r o n loadings m y be

a t t r i b u t e d t o t h e c a t a l y t i c p r o p e r t i e s of i r o n reported f o r t h i s concentration range i n t h e l i t e r a t u r e ( r e f . 17). E f f e c t of sulphur compounds Feedstocks

of

catalytic

cracking

usually c o n t a i n s e v e r a l thousand ppm

(0.3-1.0% by weight) of sulphur i n form of organic compounds. I n t h e cracking

572

r e a c t o r , t h e se compounds are p a r t i a l l y converted t o coke, and i n t h e oxidiz ing atmosphere of the regenerator t o sulphur oxides.

In order t o test t h e e f f e c t of sulphur compounds on t h e a c t i v i t y of t h e promoted cracking c a t a l y s t BF had been exposed t o 5000 ppm H2S i n

promoter,

hydrogen gas stream az 49OoC f o r 1 hour and t he n its CO-oxidation a c t i v i t y has been

measured.

activity

The

compared

catalyst to

the

did

not

show any change i n t h e CO-conversion

unsulphided

c a t a l y s t , i . e . sulphida tion does not

e f f e c t t h e a c t i v i t y of t h e promoter. The catalyst

effect

BF h a s

S02-concentrations

of

sulphur

dioxide

on CO-oxidation

a c t i v i t y of promoted

been measured by determining t h e CO-conversion at d i f f e r e n t in

the

CO/air

Sulphur dioxide causes a s l i g h t

stream.

decrease in

Re sults

are

shown i n Figure 3 .

CO-conversion. The shape of t h e

curve i n t h e SO*-concentration i n t e r v a l 0,03-3% by volume i s loga rithm ic . Over

3%by v o l . S02-content t h e CO-conversion does not decrease any more. The

decrease

a f t e r having

in

stopped

t h e CO-oxidation a c t i v i t y caused by SO2 is r e v e r s i b l e : the

SOP-flow,

CO-conversion was re -e sta blishe d, i . e .

promoter/sulphur compounds formed on the s u r fa c e are not s t a b l e and e x i s t only i n t h e presence of S02. This who proved

conclusion that

in

i s confirmed by observations of Wang e t al. ( r e f . 18.) the

presence

of

air at higher temperatures surfa c e

sulphur/noble m e t a l compounds decompose mainly t o t h e metal and metal oxide.

Fig. 3. E f f e c t of sulphur dioxide on CO-conversion a c t i v i t y of promoted f r e s h cracking c a t a l y s t BF.

573

Kinetics of coke combustion Comnercial cracking catalyst BF was mixed with different amounts of a comrcially available oxidation improving concentrate. The samples contained 0, 0,05, 0,1, 0,5 and 2,0% by weight of the concentrate. Cracking experiments were run with these samples after calcination at 700°C for 2 hours in air. The coked catalyst samples were cooled down in an inert atmosphere. Computer aided differential thermal gravimetry curves obtained in air with respect to promoter concentration with the Mettler thermobalance were used to calculate kinetic constants (reaction order, activition energy, preexponential factor) assuming the validity of the Arrhenius model (refs. 19, 20). Data obtained are shown in Table 2. As shown in Table 2., the reaction order of coke combustion in air is practically 1 , and kinetic factors do not depend on coke level of the catalyst, however values of activation energy and pre-exponential factor change with promoter concentration. The latter correlations are presented in Figure 4. Figure 5. is a plot of kinetic constants of coke oxidation vs. the onethird power of oxidation improver concentration, showing a linear correlation. This finding explains the theoretical basis of empirical correlations developed earlier (refs. 1, 10) stating that regenerability is a linear function of the one-third power of oxidation improver concentration. TABLE 2. Effect of concetration of oxidation improver on the kinetic constants of coke combustion. Promoter Coke on concentration the w t % catalyst

Reaction order

Activation energy kJ/mol

he-exponential factor In KO

wt%

0

6.08

1 .og

72.7

5.28

0.05

4.62

1.05

71.8

4.95

0.10

5.32

1 .oo

66.5

4.12

0.50

5.33

0.96

62.3

3.39

2.00

4.98

1.04

55.9

2.61

574

0

Fig. 4. Relationship between kinetic constants of coke oxidation and oxidation improver concentration.

Fig. 5. Kinetic constants vs. oxidation impover concentration.

575

ACKNOWLEEEMENT The authors wish to express their thanks to Mr. G. Pokol, Associate Professor and to the Analytical Department of Gedeon Richter Pharmaceutical Works Ltd. (Budapest) f o r their kind help in carrying out and interpreting thermal analyses.

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